1. Field of the Invention
The invention herein relates to the removal of water from corrosive gas streams. More particularly it relates to the production of substantially water-free gas streams for use in the production of semiconductors and similar products which cannot tolerate the presence of water during manufacture.
2. Description of the Prior Art
There are many products which are made by depositing elemental materials and compounds from gases or vapors containing those materials. Such gas- or vapor-deposition process are commonly used to form high purity products, such as coating and etching silicon wafers for semiconductors or other electronic substrates, semiconductor layer circuitry, or prosthetic products for human and animal usage. In order for the products to be of high purity, the gases fed to the deposition chamber must be themselves of high purity. This requirement has become increasingly important as the products, especially semiconductors and integrated circuits, have become more complex, with higher component densities, more and thinner layers and submicron conductor widths and spacings. The presence in the gases of particulates or, most importantly, water, can substantially reduce the yield of useful products from the manufacturing process, and can also cause substantial damage to the manufacturing equipment.
Water is one of the most common and yet most difficult impurities to remove from the gases. Water is of course ubiquitous in almost all ambient environments. Even systems which are nominally referred to as "dry" usually have significant amounts of water, and most drying processes can reduce the moisture content of a gas only to a "minimum" which is still in the parts per million (ppm) range. However, since for many purposes water contents in the ppm range are quite acceptable, there are numerous patents and articles in the literature dealing with such types of "ppm drying processes."
In the manufacture of many electronic products or high purity wafers, chips, or ceramics, however, moisture contents of the depositing gases which are in the ppm range are excessively wet. To form satisfactory products, the water content of the depositing gases must be reduced to the parts per billion (ppb) range, usually down to no more than about 100 ppb. See Whitlock et al, "High Purity Gases," in Ruthven, ed., ENCYCLOPEDIA OF SEPARATION TECHNOLOGY, vol. 1, pp. 987-1000 (1997).
Further, there are a number of gases used in the manufacture of high purity products which are readily handled when dry, but which in the presence of water become highly corrosive to manufacturing system equipment. Corrosion in turn causes gas leaks and component failures in valves, regulators, filters, flow controllers, tubing and fittings. Among the most common gases which are aggressive and corrosive when they have a high moisture content are the halogen gases, such as the hydrogen halides and the gaseous halides. The halogen gases have been found to be excellent silicon etchants, and therefore it is important to insure that they can be used effectively in the production of the high purity products.
The corrosive effect of the halogen gases in the presence of water not only causes damage to the equipment, it also is detrimental to the products. The water content itself causes problems in product integrity and yield. In addition, the gas-induced corrosion of the equipment, tubing, etc., generates small particles of corroded metals. These become entrained in the gas stream and are carried into the product formation chambers, where they deposit onto products being formed, thus ruining the products and requiring their rejection.
There are prior art processes which can remove water from gas streams down to the .ltoreq.100 ppb range. However, these are effective only for gases which do not react with the dehydrating material. Such gases include the conventional "inert" gases as well as other gases and vapors which are unreactive with (i.e., "inert to") the dehydrating material. Such processes, however, do not work satisfactorily when the gases are corrosive in the presence of moisture, since as noted above such gases attack the dehydrating material and quickly render it useless. These processes are therefore not useful to dehydrate the halogen-containing gases and gases of similar corrosivity.
Attempts to use silica, alumina, titanium tetrachloride, and other oxides, halides, etc. to remove moisture from halogen-containing gases have not been successful. While corrosive halogen-containing gas streams can be dehydrated for short periods of time down to the 10 ppb level, the corrosive effects of the halogen gases very quickly damage and deactivate the active dehydration materials requiring frequent removal and replacement of the dehydration materials in order to insure a feed stream of dried halogen-containing gases in the manufacturing systems.
Active metals have also been used as "getters" to remove oxygen from streams of other gases, notably argon, and there is substantial prior art regarding various gettering processes and metals. In particular, metals such as titanium and zirconium have frequently been used as getters; see for instance, U.S. Pat. Nos. 4,629,611 and 5,556,603. In such processes the oxygen is removed by reacting it with the metal to form a solid oxide which can be separated from the gas stream. Wet argon, however, is not a corrosive gas in the manner of the halogen gases.
Zeolites have been used in moisture removal processes before, but not as active dehydration agents. Rather they have been present merely as carriers or substrates for various impregnated metal getters or dehydrating catalysts, as described above. In this regard they have merely been substitutes for conventional silica, alumina and carbon substrates. Typical examples of such systems will be found in U.S. Pat. Nos. 4,853,148 and 4,925,646 (both to Tom et al.).
Consequently, the problem of removal of moisture down to .ltoreq.100 ppb from corrosive halogen-containing gases remains a significant problem in the field of production of high purity semiconductors, substrates, prosthetics, ceramics and the like. Those processes which are being used are expensive because of the very short service life of the dehydrating materials and the need for their frequent replacement. In addition, since it is difficult to determine the exact rate of deterioration of the dehydrating materials in the presence of the corrosive halogens and halides, user of such dehydrating materials must schedule their discard and replacement at intervals less than the shortest expected service life. To do otherwise would risk failure of a dehydrating unit with the resultant loss of contaminated product when the excessive moisture reaches the production chamber through the failed unit. Consequently, the current systems require that many if not most of the dehydrating units must be discarded while they still have some degree of useful service life left, thus further increasing the expense of the system operations.
In most high-purity product manufacturing processes, it has been conventional for halogen gases to be stored in and supplied from standard gas cylinders. The volume of gas in each such cylinder is of course limited, so that in larger scale manufacturing processes, it becomes necessary for process operators frequently to replace emptied cylinders and replace them with fresh, full cylinders. This frequent handling and movement of heavy, awkward gas cylinders represents a safety hazard to the operators, as well as providing opportunities for gas leakage and increasing the cost of manufacturing. Also importantly, each time an empty gas cylinder is detached from the system and a new full cylinder attached, there is an opportunity for ambient moisture to enter the system, thus increasing the dehydration load on the system and accelerating the system corrosion. The industry is beginning to require gases to be supplied in large volume containers which need to be changed only at infrequent intervals (usually measured in months rather than hours, as with the individual gas cylinders). A preferred type of large volume container is the "tube trailer," a semi-trailer which is constructed with a number of "tubes," high capacity extended high pressure vessels, which are interconnected or operate through a common manifold. A tube trailer can be parked at a manufacturing facility and attached to the gas supply system, and will typically have sufficient gas capacity to supply the halogen gas to the facility for a period of months. This eliminates the need for frequent handling and changes of conventional gas cylinders and reduces dramatically the number of times that the system needs to be opened for cylinder changes and thus exposed to ambient moisture infiltration. Equally importantly, since the tube trailer is usually parked outside the manufacturing building, it also positions the gas supply outside, so that any gas leakage does not endanger the operators and access to the leaking vessels for repair or containment is greatly simplified.